Skipper’s Tips #7 – Chart Plotter Rules, OK?

Hello all!

Please excuse my prolonged absence, I’ve been rather preoccupied with work lately and everything else has had to take a back seat.  However, I have finally managed to find the time to sit down and post a new Skipper’s Tip.

I do hope you enjoy this tip and also that you find it informative.  If you have any questions or are at all confused by what I write, then please contact me and I will try to explain.  

This tip is all about paper charts and the tool we use to extract navigational heading information from them. Electronic chart plotters are amazing pieces of technology these days and they hardly ever go wrong.  Nevertheless, we must not forget that as good as these pieces of equipment have become, they are still no substitute for knowing how to navigate ‘manually’.  You should always plan on a paper chart and always keep a paper chart with you when you are on your passage and you should make a regular note on the paper chart of your position.

The single best tip I can give you regarding navigation, is to know exactly where you are, at all times!

The Chart Plotter

When you first see a paper chart and start looking carefully at the detail, it can be a little awe-inspiring. Imagine all the work that went into collating that information.  Especially impressive, is the fact that charts have changed very little since the world was first mapped.  I have a mental image of a couple of men in naval uniform in a small boat with a long piece of string and a heavy weight, a big pad of paper and infinite patience.  Today, we take for granted the effort that must have gone into making the world’s seas and coastlines appear in 2D, so that we can safely navigate in our boats. However, if it weren’t for the efforts of these determined explorers we wouldn’t stray very far from home would we.

To use a chart effectively you need to understand a few basic rules.

  • Always measure distances from the side of the chart nearest to where you are planning to sail
  • Look at the compass rose shown on your chart to determine what the allowance for variation must be
  • All the numbers peppered across the chart are ‘charted depths’ or ‘drying heights’ and as such make no allowance for tidal height
  • These depths or drying heights could be in feet, fathoms or metres and you need to look at the key for the chart to know what you are dealing with
  • Make sure the chart is designed to be used with GPS, if that is how you intend to navigate
  • The squiggles, lights, lines, abbreviations, etc. are all explained in the Admiralty publication ’Chart 5011’
  • The tide stream data will be referred to one specific port and will be listed as ‘true’
  • In fact, everything shown on the chart is ‘true’ not ‘magnetic’
  • Pay particular attention to the scale of the chart

In order to work out which way to point the boat or indeed, to describe a line on the chart to show where we have been, we will need to employ an accurate process.

Enter the ‘chart plotter’.

There are many incarnations of the modern chart plotter, a great improvement I have to say on the ‘parallel rules’ of the past. Essentially, the one I am going to describe using in this example looks like a very large and wide rule with, in the centre, a dial that you can swivel.

There are two key things to remember with ’chart plotters’. Firstly that whenever you place the plotter on your chart, the centre dial, which spins, must always have ’N’ (North) pointing to and aligned accurately with, true North on your chart.  The second rule is that the big arrow at one end of the plotter will always indicate the direction or heading of the bearing.

If you are trying to draw a line in an easterly direction, i.e. 090 degrees and yet the arrow on the plotter is pointing west (270 degrees), then you have it upside down…  It will be really helpful if you try to think of these numbers as directions, rather than just random numbers.  000/360 is North, 090 is East, 180 is South and 270 is West.

So, the way it works, is that you lie the plotter on the chart, parallel to a line which you want to know the bearing of, spin the dial until the ‘N’ at the top aligns with North on your chart and then read the bearing figure from the dial, which lines up with the large arrow at the end of the plotter.  This will tell you the ‘true’ bearing of that line or indeed, the bearing between two points on your chart.  The next thing you will need to do, of course, is to convert this ‘true’ bearing to a ‘magnetic’ or even ‘compass’ bearing, so that you can use it with the ship’s fixed compass to navigate your course.

If you are using the chart plotter to show your course travelled on the chart, in the way you might if you were showing your passage as part of your log, then you will need to do all this is reverse. Firstly, assuming you know the direction you have been travelling in, convert that direction from compass or magnetic back to true. Then align this number with the chart plotter dial and the large direction arrow.  Lastly, place the chart plotter on the chart, being careful not to turn the dial and align the whole plotter with true North (use the N on the dial for alignment; DON’T TURN THE DIAL, TURN THE WHOLE PLOTTER) and then draw a line down the length of the plotter to show your track.

I have to say, trying to describe this process in words is not easy.  It is much easier to actual show someone this but if you sit down with my explanation, your chart plotter and a chart you will soon get the idea I’m sure.

I think my next Skipper’s Tip had better be about True, Magnetic and Compass bearings!

This has been another Captain Corbett’s Adventure.  If I’m not on Jersey teaching a private tuition Day Skipper theory or Yachtmaster theory course, then I’m either spending time with someone on their boat, giving them the confidence to take their boat out with their family and friends on board or I’m off somewhere exotic delivering a boat. Either way, I’ll write it up and put it on the Blog for you all to see, so keep popping back to see my most recent adventures.

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